Balance and tracking of rotor blades



Sept. 6, 1960 ,F. STULEN ETAL BALANCE AND TRACKING OF ROTOR BLADES Filed Oct. 21, 1957 ll 27 I FIG. 5

FIG. 6

INVENTORS. FRANK 4L. STULEN a c. BALI/.ZIER AFT STRUCTURE Z? cg. ELASTIC SPAR AXIS ATTORNEY Patented Sept. 19%0 BALANCE AND TRACKING OF ROTOR BLADES Frank L. Stolen, Traverse City, and Alb C. Ballauer, Grand Traverse County, Mich., assignors to lfarsons Corporation, Detroit, Mich., a corporation of Michigan Filed Oct. 21, 1957, Ser- No. 691,190

8 Claims. (Cl. 170159) This invention relates to the manufacture, balance and tracking of rotor blades for helicopters and the like. Its objects include matching the performance of rotor blades which make up a set, and also matching blades to a standard whereby to achieve interchangeability. More detailed purposes will be apparent from the discussion which follows.

The matching of performance characteristics of rotor blades has been one of the most persistent problems of the helicopter industry. Individual differences between rotor blades in their response to dynamic and aerodynamic forces, and deflections in bending and twisting under these forces, can cause serious difficulties, including roughness in operation and control.

To match as closely as possible the performance of the blades which constitute a set, they are usually tracked" together. It is conventionally considered sufiicient to achieve substantial correspondence merely in the tip paths of the several blades being tracked with each other; this is conventionally done by adding chordWise-adjustable tip weights.

The phenomena involved are complex and difficult and the solution proposed by the prior art cannot be summarized in any simple statement. In a very general way, it may be stated that according to the prior art, blades are individually balanced according to the designers favorite criteria, and then tracked to each other, in sets of two or three, utilizing either a whirl test stand or the rotorcraft itself. It has not heretofore been thought possible to mass produce blades which are preliminarily balanced to a chosen mass distribution and which may be used inter changeably in sets by a simple tracking procedure whereby the master blades aeroelastic performance is closely approximated.

One cause of difficulty in the prior art has been the use of concentrated weights interposed at various points along the blade for the purpose of achieving better track. Shifting of such concentrated weights from one point to another may achieve one good effect but have a secondary bad effect. For instance a concentrated weight which does not lie on the elastic axis of the blade tends to twist the blade as the blade flexes. Such twisting will be cyclic, resulting in cyclic variations in the air load distribution which have secondary effects on the relatively elastic blade structure. This is a highly simplified example of one aeroelastic effect which follows from spanwise discontinuity in mass distribution of such shiftable concentrated tracking weights.

In contrast to this conventional tracking procedure, the present invention importantly lessens the need for (and extent of) blade tracking, both by providing structure and a preliminary balancing which obtains a substantial station-by-station correspondence in blade properties. Use is made of blade structure designed for minimum variation in spanwise elastic properties, mass distribution, and center of gravity; and provision is made for bringing each blade up to a uniform standard weight and balance without disturbing the spanwise mass distribution. The factors which give rise to the need for tracking are thus minimixed. The invention further provides for tracking by utilizing one-piece leading edge and trailing edge full span balances. Initially, blades are brought up to the standard weight and are balanced at a pre-selected chordwise position near the aerodynamic center. Subsequently this position may be adjusted to remedy such tracking deficiences as the individual blade may exhibit. The standard weight is kept to a value less than the total weight of a theoretical blade made up of parts which individually were of the maximum permissible weight.

Stated somewhat more specifically (but without limiting the detailed description which follows) the present invention teaches how to construct blades which are matched to a master or standard blade in all significant respects. It teaches providing a blade with structural elements which extend continuously, full span, from root to tipsuch as a leading edge spar and an aft structure including a trailing edgewith spanwise hollow tubes along the inside of their leading and trailing edges at the level of the chord plane, as shown. The invention also teaches securing within each tube a full span balance, which may itself be a tube or a solid rod which fits closely Within one of the first mentioned tubes. Using a trailing edge balance weight represents a novel step to providing standard blade characteristics. In combination with the other features and method steps taught, each blade, mass produced out of material subject to normal tolerances, will have the same Weight and substantially the same mass distribution as the master blade; also a similar distribution of elastic characteristics such as bending and torsional stiffness and response to periodic forces. As a result, the aeroelastic responses of the blades may be also readily standardized; and for this purpose the present invention teaches how to select and use a pair of such full span balances in the leading and trailing edges of each blade to achieve balance with the master blade of weight and mass moment at ail stations (i.e. cross sections) along the span. The invention also teaches how to use the same two fixed positions for balances in the final tracking of the blade so that it Will perform like the master blade in flight, preserving in such final tracking a spanwise continuity of the blade properties which affect its flying qualities and avoiding the aimlessness of prior art tracking.

In the accompanying drawings:

Figure 1 is a plan view of a rotor blade embodying and made according to the present invention, shown somewhat schematically;

Figure 2 is an enlarged left end view of Figure 1;

Figure 3 is an enlarged sectional view taken along line 33 of Figure 1;

Figure 4 is a further enlarged fragmentary section taken along line 4-4 of Figure 2;

Figure 5 is a similarly enlarged fragmentary section taken along line 5-5 of Figure 2; and

Figure 6 is a diagrammatic sketch of a hypothetical rotor blade referred to as illustrating the method of the present lnvention.

The rotor blade show-n comprises a leading edge spar generally designated 14) which extends full span from the blade root to the blade tip. Such spar 10 comprises the principal structural member of the blade and may be made up of a continuous steel, curved leading edge plate 11, and a vertical channel 12 likewise full span and having upper and lower flanges 13, 14 brazed within the upper and lower inner surfaces of the plate 11. The steel sheet material for the leading edge plate 11 and channel 12 is commercially furnished not to a precise thickness but within a range of commercial tolerances, which accounts for a variation in weight as between one piece of sheet or plate and another. However, there is relatively little'variationin the thickness of any single sheet or plate'from one end to the other. Hence, by fabricating the spar so that its principal member extends full span, without intermediate chordwise members between the 1 root and tip, 1 therepis little I variation spanwise in the mass distribution of the spar '14).

While-the aft structure may- -likewise=consist of metal sheet elements-running -the entire; sp'an, -,there is here shown plastic molded aftstructure -generally 4iesignated 15, and'is molded .of fiberglass fabric-andresin, as hereafter described,- the fiberglass consisting 'of "sheetseach extendingfrom-the-blade root to the blade tip. =As-with steel sheets, substantial weight variation doesnot exist within a single piece -of the cloth. We -usemfamiliar manufacturing-controls to assure aneven distribution of the resin plastic. "'Intheaft' structure*- thespanwise mass distributionis thus kept substantiallyconstant along the span. 7

The aft structure 15 consists of a spanwise reinforced shell molded of fiberglass-cloth and plastic resin to the airfoil contour, and having a nose portion-16 fitted ;be tween the aft edges of-the-leading edge plate 11 and against the aft side of the-channel 12, and terminating in a trailing edge 17 reinforced by apre-molded plastic triangular insert--18 having a full span cylindrical hollow 19. A simple ground adjustable metaltrim tab- 20, bonded over a portion of the trailing edge 17, is added for matching the blades 'aerodynarnically.

Brazed ortack welded inwardand closely adjacent the leading edge plate 11 is a full span leading edge tubular receptacle 21 whose inner surface is cylindrical, preferably a thin-walled tube. This receptacle 21 serves a function similar to'the trailingedge cylindrical hollow 19, as hereafter described. At'the root, the blade is provided with a retention fitting generally designated 22 having bores 23 for mounting tothe blade hub. The retention fitting; 22 is shown somewhat schematically. 'Ihe blade may 'furtherlhaveupper and lower root reinforcement plates 24,extending rearward chordwise to grasp the trailing edge '17. Preferably the root portion of the blade aft ofthe spar is closed by a root rib 26. Figure 4 is taken'throughthe root end fragment of. the leading edge receptacle 21,jinthe root end of which is rigidly affixed a receptacle bushing .27 projecting inward from the sparllltoward t he rotor hub and externally threaded to receive a cap,nut'28. Inside the tubular receptacle 21 is inserted a full span, leading edge mass balance 29; which in the embodiment'shown is a hollow tube. The outer wall of the tube which comprises the mass balance 29 fits fiairly close to'the innerwall of'the cylindrical receptacle 21, so as to bear against it when the blade is subjected to thebending deflectionsnormally encountered-in flight. Such bending deflections are so great, compared. with the clearances, that much of the centrifugal load due to'the mass balance 29'is resisted in friction along the cylindrical inner wall of the receptacle 21. Positive security is-afEorded by brazing the inner end of the leading edge mass balance 29 Within a retaining collar-30 which .bearsagainst the end of the receptacle bushing 27, and by enclosing'the retaining collar '30-within the cap nut 28. 'If desired, a full span solid rod or wire may be substituted for'the h'o'llow tube which servesas the leading edge mass balance 29.

Along the, trailing edge a somewhat similar provision for balance is made, with details designed for lighter weight. As shown in' Figure 5, the ,full span cylindrical hollow 19 inthe trailing edge 17 is fitted at its root end with a bushing31, fitted and bonded within the premolded triangular insert 18, and having an externally threaded projecting end portion 32 fitted with a cap nut 33. Within the cylindrical hollow 19 is provided a trailing edge mass balance Wire'34 extending the full span of the blade, and brazed at its inner or root end within an axially drilled retainer 35'having a head 36 which abuts the outer end of thebushing 31 within the cap nut 33. As illustrated, the leading edge mass balance tube 29 and trailing edge mass balance wire 34 are secured within their respective tubular receptacles 21, 19 at their root ends only. The tube 29 and wire 34 are omitted from Figure 3.

The tip end of the blade structure may be closed with a tip fairing 37.

Figure 6 is included to illustrate the method of the present invention. Thenutilization='of-full-=sp an sheet material elements for both a spar and anwaft structure, without any substantial, meinbers'Qinterniedia-te thelbl'ade root and the bladetip, resultssas fol-lows: thecenters of gravity of each spanwise member, taken at successive stations along'the span, will'lie along-straight spanwise axes, shown in thetypical schematic. section of Figure 6 as cg spar and cg aft structure. For simplicity these are shown midway between the aerodynamic center and the leading edge and trailing edge respectively. The leading edge tubular receptacle MF-likewise has a straight spanwise axis, designated "a sand shown here :as lying at the fronto-f theleadingdge; andftheitrailing edge cylindrical hollow :19 has :a similar =taxis b,:ishowu here at the aft m-arginwf the trailing'edge. 1'Theaero dynamic center of thesection is =ass1nnedto he at the 25 chord point.

The weight of -therootretention ismeglected1 inithis hypothesis, and the blade is considered as i being nmade up of a spar and an-aft structure. Ifaeveryssheet element which makes up'these structuresismfmaximum thickness, their total weight would be' the maximum, taken here as 100 lbs. --A commercial thickness tolerance of 8% is assumed, making the-theoretioal rninimum weight of the blade92 lbs. 70f 'such total:-maximum weight, the. spar may rangeirom 69 to'75 -lbs.,-While the aft structure may'range fnbnf23-to'25 lbs.

Let us assume =thata blade a'ssembl'ed':oftwo such spanwise-extending parts, eachtweig'hing: precisely a standard weight- (chosen atia certain percentage ofajstandard tolerance) balances along an axis coineident' with the aerodynamic center; withoutadded weights. Then, such balance will also be achieved withoutndded' weights if both components are of the-maximum-weight,- and likewise if both components are-of the minimumweight, without any added weights in t-he leading-edge-or trailing edge. There is, statistically, little-likelihood that any one of these combinationswill occur-inroutine-rnanufacture.

Assuming the unlikely. combination that the spar is at its maximum weight of lbs. andthe=aft structure at its minimum weight of 23lbs.,-- a llbtmass distributed along the entire spanwise trailing edge axis b will balance the blade at its aerodynamic center. This unlikely combination results in a-total-weight of=991 1bs.,--instead of the IOU-lb. weight'if both components -wereof the maximum possible thickness. maximum weight of 25'1bs. andthe spar atits minimum weight of -69 lbs., a w'eightof'3 lbs. added-along ithe leading edge axisa would balance thehlade at"the -aero dynamic centerand=bringthetotal weight to- 97 lbs. ll each component is of average 'weigh'g the combination would balance at the aerodynamic center without added weight, and such combination would wei gh 961bs.

Applying thelawsof-probabilityto theassembly of these two components,.only.a verymsmalhpercentage of blades to be assembled could exceed can arbitrarily selected'weight of say 98 lbs. This figure maybe selected as the standard weight forsuch a blade. ,-Although a statistically few blades may be assembled which cannot be balanced at theaerodynamic center-.atisuch a-weight, such few blades maybe either rejected -orbalanced at a slightly higher weight. Alternatively, rather than assembling parts. at. random, parts may be :first weighed and then selectively assembled so that vtheir. weights total less than'the standard blade (for example, by. using only one abovevaverage-wei'ghtpartper assembly),*and prac- If the eto-structure were its tically an blades so assembled will be found to be balanceable at the standard weight.

The details of procedure for balancing may be varied; but the method is essentially as follows: The mass moment of a blade of standard weight and assumed standard properties is first determined in reference to a spanwise axis, which may be either the leading edge receptacle axis a or the trailing edge receptacle axis b. This figure is used as a standard mass moment about such axis. It will normally exceed the mass moment of the assembled production blades. Each production blade is brought up to a corresponding mass moment about its corresponding axis. A simple means of so doing is to suspend each blade so that it may rotate about such axis, measure its mass moment thereabout, and then add a spanwise-distributed weight along the axis through the hollow at the opposite edge, suificient in amount that its mass moment about the first axis remedies its deficiency in mass moment as compared with the standard mass moment about such axis.

Into the hollow about whose axis the mass moment was taken, there is then inserted a sufficient spanwisedistributed weight to bring up the weight of the blade to the standard weight. This will also serve to fix the center of gravity of the blade at the same chordwise location as in the standard blade. Thisline may advantageously correspond to the aerodynamic axis of the blade, especially if the elastic axis of the blade does not depart materially therefrom.

Such balanced construction will be found substantially to lessen the need for tracking of blades. Final tracking may be by the use of small but otherwise conventional tip weights, if desired. However, without using such tip Weights, the blades may be tracked, preferably with a chosen standard blade, on a whirl test stand, using the following procedure:

With the blades in flat pitch, the trim tab 20 of the production blade is bent and any pitch link thereon is adjusted until satisfactory tracking is obtained. The production blade is then considered as aerodynamically similar to the standard blade. The manner of construction described yields, in addition, similarity as respects mass moment and center of gravity at each station along the span.

Despite these similarities, individual blades will exhibit some variation in performance characteristics, due perhaps to differences in elastic characteristics. In order to remedy these variations, the combined weight of the particular leading edge mass balance 29 and trailing edge balance wire 34, which brought the particular blade to balance at master weight, is first noted. These balances are removed, and for them substitution is made, experimentally, of other pairs of spanwise-extending masses, whose total weight equals their total, but one mass being slightly heavier and the other slightly lighter. The blades are again whirled and the track of the produc tion blade as against the standard blade is compared over a desired range of pitch angles, the experimental substitution being continued until satisfactory tracking is achieved.

The weight of the leading edge mass balance 29 may be varied by choosing a tube of different inner diameter without affecting its clearance within the leading edge receptacle 21, or by using a full span rod or stiff wire. Similarly, for the balance wire 34 there may be substituted in the trailing edge cylindrical hollow 19 any other full span mass which may be accommodated therein.

The success of this procedure in obtaining good track is thought to be explainable as follows: Where there is a departure in fact between the elastic axis and the aerodynamic center, as shown in Figure 6, the change of the division of weight between the full span balances without changing the total weight in effect tunes the dynamic and aerodynamic characteristics of the individual blade, adjusting the position of the center of gravity chordwise at each station along the span to an axis which the track ing procedure itself ,has proved to be they best center of gravity location for the particular blade. The effects on blade twist of centrifugal forces due to chordwise distribution of masses, have thus aided in achieving tracking; no isolated masses can cause undesirable dynamic, aerodynamic and aeroelastic effects; and the master weight of the blade is maintained. I

Utilizing the structural principles here described and the methods of obtaining balance and tracking, there is achieved a rotating blade wherein each airfoil portion, section by section, is in substantial trim with the aerodynamic, static and dynamic forces for which it itself is responsible. This enables the mass production of interchangeable blades, weighing less than the maximum predicted weight for the material, and possessing a uniformity of performance characteristics not heretofore achieved.

When used in the claims, the term full span is to be understood as meaning the blade span length less such length as may be provided for fittings, fairings and at tachments at the root and tip.

We claim:

1. A rotor blade comprising blade structure composed of constant gauge sheet material elements extending spanwise and uninterrupted between the blade root and tip, tubes inwardly adjacent both the leading edge and the trailing edge of the blade, balances within the said leading edge and trailing edge tubes, each of said balances extending from the blade root to the tip and means for removably mounting said balances within said tubes, whereby the total weight of said full span balances may be allocated between the said tubes.

2. A rotor blade as defined in claim 1, the leading edge tube being cylindrical, the balance therein being a tube whose outer diameter fits closely inward of the inner diameter of the leading edge tube.

3. A rotor blade characterized by spanwise continuity of structure and mass balance, comprising a full span leading edge part having a full spanwise-extending hollow inwardly adjacent the chordwise forward portion thereof, a full span aft part having a trailing edge and a full spanwise-extending hollow inwardly thereadjacent, a full span leading edge balance mounted within said leading edge hollow, and a full span trailing edge balance mounted within said trailing edge hollow, each of said balances having a spanwise-extending axis.

4. A rotor blade characterized by spanwise continuity of structure and mass balance, comprising a root retention fitting, a leading edge part secured at its root end thereto, said part including full span sheet material and having a full spanwise-extending tubular hollow inwardly adjacent the chordwise forward portion thereof, an aft part including full span sheet material and having a trailing edge and a full spanwise-extending tubular hollow inwardly thereadjacent, a rod-like, full span leading edge balance mounted at its root end within said leading edge hollow with its outer surface fitting closely within the inner surface of said hollow, a rod-like, full span trailing edge balance mounted at its root end within said trailing edge hollow with its outer surface fitting therewithin, each of said rod-like balances having a spanwise-extending axis.

5. The method of selectively assembling and balancing a rotor blade about a spanwise axis and simultaneously increasing the weight to correspond to the balance and Weight of a similar standard blade, comprising the steps of selecting for assembly, parts including a leading edge spar and a strailing edge structure whose weights total less than those of such standard blade, then assembling the blade including said selected parts, then selecting and inserting within one of said parts, adjacent its edge, a full span mass sufficient to increase the mass moment of said blade about a spanwise axis within and adjacent the edges of said other part to a value equal to the mass moment of such a standard blade about its corresponding 7 axis, and thenselecting and inserting into said otherpart a' tha i axis W h i edge, a a smia t. l spIan massls'uflicient ,to bringthe total. weight. up, to,tha't ofjsuchstandardfb l ader a' qd v lanit aatbfar ade.- ii a; spanw'is e'. axis. and simultaneously increasing the: weight thereoffto substantially, to the balance weight of'a' standard blade, comgrisi g thestegs of in; serting in a .spanwise hollow having an axisinwardly adjacent afirst edge of blade a; spanwis ,xtending:

masssufficientto increasethemassmomentof said blade about asecondspanwiseaxis inwardly; adjacent the other,

edge thereof to: alvalue equal to the massmomentabout such. secondfa xis, of. suchstandard blade, and then insert; ing'in a hollow along such second spanwiseaxis, aspan wis'e mass sufficient to bring. the total weight up to that.

of 'such standard blade. g g

7. The method of balancing and tracking a rotor blade whereby tot bringqits performancezinto close correspondence. withthat of a. blade selected, as a standard blade, comprisingtlfe steps. of; inserting in a spanwise hollow. having an axis inwardly adjuien t a:first edge of saidblade,

a first spanwise-extending mass sufiicient toincrease the mass. morne-nttof said blade-about a second spanwise axis inwardly adjacent the other edge thereof to a value equal to'the mass moment about such second aXis ofrsuchstandard 'blade, then inserting. in a hollowalong suchsec ondspanwise axis, a..second.spanwise mass sufficient to bring the total weight u to,thatoffsuch-standard blade, then whirling, the, blade together with the standard blade and'trimmin'git aerodynamically to-track with-the standardfiblade at substantially fiat. pitch, and then whirling it with the standard blade at various pitch angles and experimentally. substituting ,for said first and second spanwise masses other pairs. of spanwise-extending, masses 5 Whose total wei ghtvequals theweight of saidrfirst andsec- 0nd, masses, untiloptimumtracking with such standard.

blades is obtained over aQdieSiredtrange of, pitch angles;

8- Ih m th st t l nsi fis dms imr a a whereby to .bringj perfdrmance .mto, cmsecior'resmm;

ence with aist'andard; comprising the steps} otfinserjting iii.

an, inwardlly adjacent,;a,,'

seeondm optimum tracking is' obtained'c'ivefrj aj desired ra g 'of fiit ch angles.

References-Cited iirthe file 'ofthis patent UNITED STATESi PATENTS 

